1. Field of the Invention
This invention generally relates to digital communications and, more particularly to a system and method for controlling the flow of information in an asymmetric channel environment.
2. Description of the Related Art
As noted in U.S. Pat. No. 7,050,394, communicating over a network often involves a variety of tasks. For example, to send content (e.g., a web-page, e-mail, streaming video, etc.) from one device to another, the content is typically divided into portions carried by different packets. An individual packet includes a payload that stores some portion of the content being sent and a header that includes data used in delivering the packet to its destination. By analogy, the packet's payload is much like a letter being mailed while the header stores information (e.g., a network destination address) that appears on the envelope.
A typical router contains a line card for receiving data packets on one end, performing necessary conversions and sending out the packets at the other end. Among other components, line cards include a framer for framing/de-framing data packets, and a processor for performing protocol conversion and for controlling packet traffic. The framer communicates with the processor using a protocol such as SPI-3 or SPI-4 (system packet interface), which defines packet and cell transfer standards between a physical layer device (i.e., the framer) and a link layer device (i.e., the processor).
Generally, before transmission, a framer maps one or more packets (or packet portions) into a logical organization of bits known as a frame. In addition to packet data, a frame often includes flags (e.g., start and end of frame flags), a frame checksum that enables a receiver to determine whether transmission errors occurred, and so forth. The framer feeds frame bits to one or more devices that generate signals to be carried over a network connection. For example, for an optic signal, the framer feeds a serializer/deserializer (SERDES) and transceiver that generates optic signals representing the digital data of a frame.
Processing a received frame generally proceeds in the reverse of the process described above. That is, a device physically receives signals over a network connection, determines bit values corresponding to the signals, and passes the bits to a framer. The framer identifies frames within the bit stream and can extract packets stored within the frames.
In network terminology, the components described above perform tasks associated with different layers of a network communication “protocol stack.” For example, the bottom layer, often known as the “physical layer”, handles the physical generation and reception of signals. The “link layer” includes tasks associated with framing. Above the physical and link layers are layers that process packets (the “network layer”) and coordinate communication between end-points (the “transport layer”). Above the transport layer sits the “application layer” that processes the content communicated.
Under-run and overrun are two common problems associated with the framing of data. Overrun involves the sending of too much data, or data at too high of a rate. In this case, data sent to the framer is lost before it can be buffered, which requires that the data be resent. Under-run is associated with sending too little data, or data at too slow of a rate. Some messaging protocols, such as Ethernet, are sensitive to under-run. Ethernet frames are only transmitted if they are “full” of data. Therefore, the transmission of entire Ethernet frames can be delayed as a result of under-run. Conventionally, the use of polling messages, which is a form of handshaking, addresses the overrun problem.
It would be advantageous if a framer or physical layer (PHY) device could be more efficiently supplied with information, to prevent under-run.
It would be advantageous if the different channels of a PHY device could be supplied with data at their optimal channel bandwidth to reduce the overall system latency.